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just read your post, my teacher Mr Coyle about 8th dan I think last time we met and trained together was about 5' 6'' and 120lbs not to mention his senior years, was fast, fluid and full of beans even when training with one of his dan grades who was 6'+ 250lbs
the only question I have is do you know what its like to get hit by a guy twice your size and/or weight? reality check!
We all come in different shapes and sizes, Aikido that works for you might not work for someone else, I would suggest that you stay the same, dont put on extra muscle because you will lose it as you get older and so you will have to re adapt your Aikido.
by staying as your genetic make up has designed you, what you learn now will never need to be adapted in your later years but hey thats just my opinion. Im no expert.